During the drilling of deep holes, such as for oil and gas wells, it is often desired to transmit signals from deep within the drilled hole to surface detectors. The type of information which is desired from the bottom of the borehole includes data about the drilling conditions, such as the weight or pressure on the bit, the location of the bit, and its instantaneous angle. In addition, other significant information, such as the seismic energy input by the drill bit to the earth for purposes of seismic processing, could be utilized if the data could be efficiently transmitted to the surface in real-time, during the drilling operation.
Prior methods have been used for the transmission of data from the bottom of the borehole to the surface. One of these techniques transmits data by modulation of the pressure of the drilling fluid, or "mud", in the drill string (see Honeybourne, "Measurement While Drilling", Symposium on the 75th Anniversary of the Oil Technology Course at the Royal School of Mines (1988)). As described in this presentation, data may be transmitted by way of positive or negative changes in the pressure of the drilling fluid, accomplished by restriction or venting of the fluid to modulate the pressure. Another technique for such data transmission via the drilling fluid generates a carrier wave in the fluid which is modulated by way of a motor, or mud siren. While these systems have been shown to be operable to transmit data, they each are quite limited in their rate of signal transfer (i.e., bits per second, or baud rate) from downhole to the surface.
As well bores lengthen, the problem becomes even more severe. Under the best of conditions, the data transfer rate for the mud transmission method, for example, is on the order of 3-10 data bits per second. In practice, the usual physical conditions encountered in the drilling process, together with other problems such as wear, reduce the effective data transfer rate for this system to a rate on the order of one bit every two seconds. This limited data rate prevents the transmission of anything but rudimentary and slowly changing information in a real-time manner, for example temperature and gamma ray emission data.
In addition, this technique requires that the drilling mud continue to flow in a controlled manner during data transmission. Due to blockages and other drill string problems, the free flow of the drilling fluid cannot be guaranteed. Accordingly, use of modulations of the pressure of the drilling mud to transmit data during drilling may be limited.
Other conventional techniques include hardwire communication, either by use of conductive drill pipe or by a cable guide system, and also electromagnetic transmission using the earth as the transmission medium. Hardwire transmission requires relatively complex hardware, as either a cable system must be included in the drill string, and lengthened during drilling or the composition of the drill pipe must be selected and used, each of which can be costly. The electromagnetic transmission system depends on such factors as the conductivity of the earth, and accordingly has not gained widespread acceptance.
The transmission of data by way of vibrations in the drill pipe string has been developed to overcome some of the problems with transmission through drilling mud, hardwire, or through the earth. A first example of such a system uses a solenoid-striker which is energized electrically according to the data to be transmitted by mechanical vibrations from downhole to the surface. An example of such a system is described in U.S. Pat. No. 4,597,067, issued Jun. 24, 1986. However, it should be noted that such prior systems similarly operate at relatively low data rates (on the order of 1 to 10 Hz).
As described in U.S. Pat. No. 4,597,067, an alternative method for dealing with such low data rates is to store the detected information in a downhole memory, for transmission to the surface at a later time. While such a system provides for eventual transmission of a significant amount of data, the downhole storage negates the ability of the system to transmit data real-time. Accordingly, this type of system has limited applications, particularly when real-time information is desired, such as, for example, transmittal of drill inclination and direction data for adjustment of the drill bit during the drilling operation. In such a case, the information is valuable only if quickly transmitted to the surface. In addition, such a system also requires relatively complex electronics for its implementation.
Besides low data rate, another problem with prior systems for communicating data by mechanical drill string vibrations is the weakness of the mechanical signal, especially considered relative to the vibrations in the drill string which are inherent during the drilling operation. Accordingly, various prior techniques have used such transmission of data during such times as the drilling is stopped. Examples of such systems are described in U.S. Pat. No. 4,597,067 discussed above, and in U.S. Pat. No. 4,314,365 issued Feb. 2, 1982. While sending data during non-drilling periods provides for improved signal-to-noise ratio, such systems will of course not be useful in providing real-time information during drilling.
The problem of weak vibrational signal was also addressed in U.S. Pat. No. 4,562,559, issued Dec. 31, 1985. This method uses a series of repeaters along the length of the drill string, such repeaters providing a boosted vibrational signal (at a different frequency to avoid interference), in order to send a sufficiently strong signal to the surface. Such a system, besides adding complexity, also requires the addition of more repeaters as the depth of the borehole increases.
Another type of prior system monitors, at the surface, the inherent vibrations of the drill string generated from the drilling operation. From these vibrations, which of course depend upon the operation and condition of the drill bit, cal,culations are made as to what is occurring below ground. This method requires assumptions about the type of drill bit behavior which causes the sensed vibrations, and accordingly the deductions can be at best an approximation of the drill bit status.
Yet another prior technique utilizes a magnetostrictive device to modulate the inherent vibrations of the drilling operation, with torsional vibrations corresponding to the information to be communicated to the surface. This technique is described in U.S. Pat. No. 3,790,930 issued Feb. 5, 1974, in U.S. Pat. No. 4,001,773 issued Jan. 4, 1977, and by Squire and Whitehouse, "A New Approach to Drill-String Acoustic Telemetry", paper SPE 8340, presented to the 54th Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers (1979). The ability of the magnetostrictive device to generate mechanical signals of significant power is questionable, however.
Another system for transmitting data along a drill string is referred to as stress wave telemetry. A stress wave telemetry system is disclosed in copending U.S. patent application Ser. No. 183,231 filed Apr. 21, 1988, also assigned to Atlantic Richfield Company, and incorporated herein by this reference. This system uses equipment outside the periphery of the drill string, such as solenoids and eccentric motors, to vibrate the drill string in a manner corresponding to the desired data.
Various problems must be overcome in constructing and using a bit-to-surface data transmission having real time capability, regardless of the technique. The drilling environment is generally hostile in temperature and pressure, and also varies from location to location. In addition, the drill pipe and bit are continually moving within and against a confined space, providing vibrational noise and wear. Furthermore, due to the confined space and the distance from the surface, the ability to apply electrical power near the end of a drill string is limited.
A prior physical actuator is described in Technical Publication 165 by Philips Export B.V. (1985). This actuator is a piezoelectric actuator formed of a pile of piezoelectric discs, for example formed of lead titanate or lead zirconate, which are held in compression by a cylindrical steel spring, and which are interleaved with copper foil electrodes. Physical actuation is accomplished by application of a voltage across the electrodes which cause the discs to expand, stretching the spring and extending the actuator.
Tests of this actuator demonstrated, however, that the size of the actuator (length on the order of 50 to 100 mm) caused the amplitude of the vibrations to be too small for application to a drill string or other similarly large physical structure. In addition, it was found that the actuator, particularly the HPA2 model, would not be capable of reliably providing oscillatory motion, as the heat generated in the material during such operation would be excessive. This material is instead best adapted to operation in a similar manner as a solenoid actuator (i.e., in a DC switching manner).
It is therefore an object of my invention to provide a mechanism for transmitting data in the form of torsional vibrations along a structure such as a drill string at a high data rate.
It is a further object of my invention to provide such a mechanism which operates at a frequency and amplitude which allows its use real-time to vibrate a large scale structure, such as a drill string during a drilling operation.
It is a further object of my invention to provide such a mechanism which provides a signal having sufficient power so that significant additional hardware along the drill string is not required.
It is a further object of my invention to provide such a mechanism which fits within the circumference of the drill string, so that the drilling operation and the mechanism itself are not adversely affected by its presence.
It is a further object of my invention to provide such a mechanism which can generate such vibrations at tunable frequencies.
It is a further object of my invention to provide such a mechanism which can transmit vibrational information over a drill string as it is lengthened during drilling, without requiring modification.
It is a further object of my invention to provide such a mechanism which can be used for data transmission in the application of a drilling operation, either from bottom to top or from top to bottom.
It is a further object of my invention to provide such a mechanism which can be used at a plurality of frequencies, with the frequency response of the structure indicative of its physical condition.
Other objects and advantages of the invention will be apparent to those of ordinary skill in the art having reference to this specification together with the drawings.